Category: Visual Effects

One of the crucial task of a Visual Effects supervisor on set is to shot spherical degree high dynamic range (HDR) panoramas. Those panoramas can be very useful in post production as a source of light or just a good reference for computer generated environments. This article goes through all the steps required to create basic HDRIs.

Shooting process

As a professional you have to be consistent with your results. That why it is very important to have a clearly defined procedure in place. This video will explain what to check before shooting and how to properly shoot HDRIs

Shoot set of picture with different exposure in 4 direction (90 degree angle)

Assuming that you use 8 mm lens on full frame camera. You have to end up with overlapping pictures for each direction

Depending on your goal you can shoot between 3-5 pictures for each direction with 1 or 2 f-stop difference.

Slate

It is a good practice to shoot a slate for every panorama that you shooting. The reason for that is when you stiching your photos it might be difficult to find where is one sequence ends and another begins. An example of a slate by using a smartphone illustrated on the Figure 5.

Slate for each panorama should include:

Project name

Scene (e.g. Forest, Silver Lake, Garage Interior etc.)

Point Number (Different shooting point within the same scene)

Figure 5: Example of Android slate

Stitching images together

I would recommend PTGui for stitching your images. It does fantastic job combining multiple exposure Canon RAW images and then stitching it together.
If you did not messed up alignment or image exposure along the way this process should be automatic.

Everyone who ever worked on integration of computer generated (CG) image over a life action video knows unpremult/premult workflow. The main ruke is before applying any color corrections to CG render it must be divided by alpha channel (unmultiplied). The whole thing about pre-multiplication and transparency is quite simple.

What in Nuke jargon sounds like “premult” in normal math language is

newRGB = RGB * Alpha

Similar for “unpremul”

newRGB = RGB / Alpha

The problem occur when you get footage which is already premultiplied and you multiply your RGB channel by Alfa again. In fact that exactly what happening when you try to use some footage with Alfa channel as transparency in Maya.

If you think your image is Unpremultiplied (but it’s really Premultiplied) and you add a Premult node you basically multiply the image twice (RGB x Alpha x Alpha). If you have an RGB pixel of 0.6 and you multiply it by the alpha pixel of 0.4 you get a correct value of 0.24 for that RGB pixel. But if you premultiply the image twice you are effectively doing 0.6 x 0.4 x 0.4 giving you a value of 0.096, which darkens the edge.

There are situations when you will want to Unpremultiply an image. The general rule for any form of colour correction is: Unpremult the image first, do the colour correction and then Premult back to its correct state. This is so you don’t accidentally colour correct any of the transparent Alpha edges. [2]

Linear workflow is the subjects of a lot of mysteries. Indeed, it is convoluted an hard to understand especially if there are multiple ways to achieve similar but not the same result.
Linear workflow mean that all your images going trough the pipiline in linear color space. For example render output from Maya in linear space go to Nuke where get processed and render to final delivery format in sRGB.
In this article I want to show my observation end experiments that I’ve done with V-Ray and Maya and how at firs glance similar things can produce quite different result.

The most important thing to know is that all of the renders perform calculation with linear colors, thus all the sources supplied, such as texture images, has to be linear. People often forget that shaders also need to be supplied with linear images.

Most of the source files (.jpg, png, tif, etc.) used as textures have non-linear gamma (2.2) applied to it.

Majority of render engines perform texture filtering in two places. First, when a shader process a texture, second, when the render engine calculating the image (anti-aliasing). As a matter of fact, we want to convert our images before shader start to perform its calculations, otherwise when it come to applying filtering the math will be wrong.

When you select you gamma conversion in application there are usually a couple choices sRGB and Gamma 2.2. This color spaces have slightly different curve which probably will be undistingvishble. Here is an example of two overlapping curve taken from Nuke LUT Settings.

My test set up include two texture images. One texture used as is with gamma 2.2 color space, another was converted to linear by using Nuke Colorspace node.

First I went and disabled all of the filtering on texture as well as in render settings in order to get clean more predictable result. In term of linear workflow I’ve been testing the following cases:

Input

Option

Case 1

Texture in gamma 2.2

Linear Workflow Toggle On

Case 2

Texture in gamma 2.2

Vray Texture Input Gamma 2.2

Case 3

Linear Texture

All off

Here is example with Vray Texture Input Gamma 2.2.

This image show Linear Workflow Toggle On.

With linear workflow toggle ON you will get warning saying that linear workflow toggle was deprecated but documentation doesn’t explain why, I also tried to find information on Chaos Group forum with no luck.

Linear workflow– this option is deprecated and will be removed in future versions of V-Ray. When this option is checked V-Ray will automatically apply the inverse of the Gamma correction that you have set in theGammafield to allVRayMtlmaterials in your scene.Note: this option is intended to be used only for quickly converting old scenes which are not set up with proper linear workflow in mind. This option is not a replacement for proper linear workflow.http://docs.chaosgroup.com/display/VRAY3/Color+Mapping#

The following image demonstrate the difference between Linear Workflow Toggle and on texture gamma correction. One main thing to note here that image corrected with VRay Extra Attribute (Case 2) has now difference with linear image without any correction (Case 3). On the other hand, image witch rendered with linear workflow toggle on has noticeable degradation in dark colors.

Most likely degradation happening due to filtering process that happening when texture get process by shader. By specifying texture input gamma on shader VRay can interpret texture to linear work space before doing the actual calculation. In case when linear workflow is turned on in render settings the texture gets filtered with non-linear gamma which lead to some errors in shader math and only after that render convert the texture to linear work space and perform further calculations.

Resources

This article was inspired by the great tutorial from Alexey Mazurenko. Unfortunately only available in Russian.https://vimeo.com/53806660

Python Filtering technic described in the same post by King Tapir work pretty well and do not required that all of your material assigned on the object level. However, it might be difficult to figure out how to use it for the first time.

Made a .py file. In my case it called filter.py and contain following code.

Called just prior to the ray_end statement which locks off the settings for an instance object. The function can query object: settings and possibly alter them.

object:surface = ('')

The surface shader attached to the object.

vm_overridedetail / object:overridedetail = ('false')

When geometry has shaders defined on a per-primitive basis, this parameter will override these shaders and use only the object’s shader. This is useful when performing matte shading on objects.
Not supported for per-primitive material assignment (material SOP).

'op:/shop/AO'

Shop material that we going to override

Tell mantra to use this file as a Python Filter

Also, don’t forget to force writing of all shaders into .idf by setting Declare All SHOPs.

This task was bothering me for a long time. We have educational license of Houdini in our school which works without any limitation within the school network. But often I want to reuse my asset created in school for other projects, however an attempt to do so transform my .hip file into apprentice version with all of the limitations applied.

Our post-production facility recessives DPX plates with .3dl LUT file from DaVinci Resolve. The LUTs were for two different cameras one of which is ARRI Alexa and another is RED. The task is to apply this LUTs to every dailies rendered by all artists.

Solution

First, we need to create a gizmo for each LUT which contain Vector Field Nuke node with the following expression in the “fectorfield file” field.

[getenv NUKE_LUT]/A_CAM_ArriLogC.3dl

where “getenv NUKE_LUT” will grab session environmental variable with name “NUKE_LUT” which contain path to the LUTs folder.